As generally discussed in U.S. Pat. No. 5,186,284 (Lamela et al.) and U.S. Pat. No. 5,409,303 (Engelbert et al.) and as otherwise commonly known, spring-set, pressure-released brakes are often used on over-the-highway or off-road vehicles as parking and/or emergency brakes. U.S. Pat. No. 5,203,616 (Johnson) describes an electro-hydraulic parking brake control system (involving a seemingly-complex circuit) in which the spring-applied parking brake is set if the transmission shift lever is in "Park" or if the vehicle is shut down and motionless, either event de-energizing the parking brake control valve solenoid. A check valve is said to insure that the brake release chamber is not sumped in the event a particular pump fails.
Brakes of the spring-set, pressure-released type are particularly well suited for parking or emergency purposes since braking is caused by spring force rather than by hydraulic or pneumatic pressure as with the vehicle service brakes. The rationale is that while pressure-operated service brakes are highly reliable, a spring-applied brake is even more so.
But spring-applied, pressure-released brakes are not without difficulty. Brake-releasing fluid under pressure is required to be continuously applied to the brake in order to keep the brake released. Such fluid is often obtained from a hydraulic or pneumatic control line, the pressure in which is regulated at a rather low level, e.g., 300 psi. Other vehicle systems, e.g., shifting transmissions, may also rely for control pressure upon the same line.
When, for example, a transmission is shifted by applying fluid pressure to one or more of the transmission shift mechanisms, the resulting demand on the control line may cause the control line pressure and, notably, the brake pressure to momentarily dip well below the regulated pressure. It is not unusual for a spring-applied brake to momentarily set during such a dip in pressure or to at least start to set. The resulting brake "grab," however brief, is quite noticeable to the operator.
A new, simpler brake control system and a new method which improve system reliability and which are suitable for use with spring-set, pressure-released brakes would be a notable advance in the art.